Merkel cell carcinoma (MCC) is an aggressive neuroendocrine skin cancer that develops on sun-exposed areas in individuals who are over 50 years of age or immunosuppressed. In 2008, genetic analysis of MCC tumors revealed the existence of a previously unidentified virus, dubbed Merkel carcinoma polyomvirus (MCV). MCV DNA was shown to be clonally integrated into the host cell genome in many MCC tumors. The finding suggests that MCV is an etiologic factor underlying the development of MCC. To begin to address the biology of MCV, we developed viral reporter vectors based on the MCV capsid proteins. We have also developed reporter vectors based on better-studied human polyomaviruses, such as B-K polyomavirus (BKV). Our first application of these new reporter vectors was to quantitate MCV-specific serum antibody responses in human subjects. We found that 88% of a population of healthy adults displayed MCV-specific antibody responses, suggesting that infection with the virus is very common. Interestingly, our analysis revealed that patients with MCC displayed dramatically stronger antibody responses against MCV than control subjects. This suggests that strong MCV-specific antibody responses may serve as a biomarker for MCV-associated diseases. In a separate line of work, we discovered that healthy subjects shed MCV virions from the surface of their skin. The project involved the development of cutting edge methods for random amplification and deep sequencing of the genomic DNA of polyomaviruses, papillomaviruses and other skin-surface microbial DNA. The project led to the discovery of two novel polyomaviruses, human polyomavirus 6 (HPyV6) and HPyV7. More recently we discovered HPyV10 and confirmed the existence of HPyV11. It is not yet clear whether the novel viruses cause any human diseases. However, HPyV6 can be found in skin cancers arising on patients treated with BRAF inhibitors and HPyV7 has recently been found in skin rashes. The remarkable recent success of virus-like particle (VLP) based vaccines against HPV suggests that similar VLP vaccines against polyomaviruses might be similarly effective. Thus, if our work can uncover causal connections between MCV or other HPyVs and major human diseases, such as cancer, it might be possible to rapidly develop a vaccine for preventing any diseases the polyomaviruses might cause. In recent years, our lab has shifted focus toward BK and JC polyomaviruses, which commonly infect the human urinary tract. We are seeking to understand the basic mechanisms through which these two pathogenic human polyomaviruses infect cells and elucidate the mechanisms of antibody-mediated neutralization.